Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.

South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion

Crustal thickness mapping of the central South Atlantic and the geodynamic development of the Rio Grande Rise and Walvis Ridge

Eocene continental breakup in Baffin Bay

This article analyzes topographical and geological settings in the Japan Archipelago for comparative raster data processing using GRASS GIS. Data include bathymetric and geological grids in NetCDF format: GEBCO, EMAG2, GlobSed, marine free-air gravity anomaly and EGM96. Data were imported to GRASS by gdalwarp utility of GDAL and projected via PROJ library. Methodology includes data processing (projecting and import), mapping and spatial analysis. Visualization was done by shell scripting using a sequence of GRASS modules: ‘d.shade’ for relief mapping, ‘r.slope.aspect’; for modelling based on DEM, ‘r.contour’ for plotting isolines, ‘r.mapcalc’ for classification, ‘r.category’ for associating labels, and auxiliary modules (d.vect, d.rast, d.grid, d.legend).The results of the geophysical visualization show that marine free-air gravitational anomalies vary in the Sea of Japan (–30 to above 40 mGal) reflecting density inhomogeneities of the tectonic structure, and correlating with the geological structure of the seafloor. Dominating values of geoid model are 30–45 m reflecting West Pacific rise, determined by deep density inhomogeneities associated with the mantle convention. Sediment thickness varies across the sea reflecting its geological development with density of 2 km in its deepest part and thinner in central part (Yamato Rise). The aspect map and reclassified map express gradient of the steepest descent.

/pdf/noaa-marine-geophysical-data-and-a-gebco-grid-for-the-4yh9999o5h.pdf

NOAA Marine Geophysical Data and a GEBCO Grid for the Topographical Analysis of Japanese Archipelago by Means of GRASS GIS and GDAL Library

Abstract Thick sedimentary cover (≤16 km), vintage seismic and disparate crustal terranes have hindered understanding of the basement underlying the Caribbean plate. The plate formed by Early Cretaceous to Miocene amalgamation of four crustal types: the Caribbean Large Igneous Province oceanic plateau; the Chortis continental block; the related Great Arc of the Caribbean and Siuna/Mesquito Composite Oceanic Terranes island arc blocks; and the Colombian and Venezuelan basin oceanic crust. We characterize each terrane through interpretation of surface geology, 62 000-line-km of 2D seismic reflection data, 366 seismic refraction stations, 47 wells, 74 basement samples, 2D forward modelling, magnetic and gravity anomaly grids, and integration of previous studies. Basins overlying island arc crust are small, fault-bounded and deep, while on continental crust, they are broader and shallower. Strongly flexed oceanic and oceanic plateau crust along amagmatic subduction zones on the southern and northeastern edges of the Caribbean plate produce the largest and deepest sediment-filled basins. Areas of proven hydrocarbon source rocks and mapped seeps are associated with continental and island arc terranes in the western Caribbean plate, while organically-rich, but immature, Late Cretaceous source rocks occur across the more elevated areas of the central and eastern Caribbean plate interior.

Tectonic terranes underlying the present-day Caribbean plate: their tectonic origin, sedimentary thickness, subsidence histories and regional controls on hydrocarbon resources

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017GC007280

Derivation and Error Analysis of the Earth Magnetic Anomaly Grid at 2 arc min Resolution Version 3 (EMAG2v3)

Magnetic navigation requires the comparison of onboard magnetic sensor measurements to reference magnetic maps and models. Magnetic maps and models are available at a wide range of scales and scope. At any given location, measurements of the ambient magnetic field are the sum of various components, including sources internal and external to the Earth. This paper discusses maps and models of the internal components sourced in the core and lithosphere of the Earth. The long-wavelength, slowly-time-varying core field of the Earth is represented by global, satellite-based models. The time-invariant lithospheric magnetic field is represented at its longest wavelengths by spherical harmonic models based on data from the CHAMP and SWARM satellite missions and at resolutions smaller than ~300 km by grid products based on the processing of airborne, marine, and ground surveys. Regional to local magnetic anomaly maps and grids are available for selected areas at resolutions down to 100 m. Evaluation of possible map/model performance for a given magnetic navigation scenario requires consideration of amplitude and frequency of magnetic field variations, and magnetic map/model uncertainty. We present a basic analysis of magnetic navigation signals as a function of altitude and velocity.

B. Meyer

Papers

Derivation and Error Analysis of the Earth Magnetic Anomaly Grid at 2 arc min Resolution Version 3 (EMAG2v3)

Magnetic Maps and Models for Alternative Navigation